1. Field of the Invention
The present invention relates to a method for controlling the ink guidance in a printing machine, and more particularly, to a method for controlling the color ink guidance in a sheet fed offset printing machine.
2. Discussion of the Prior Art
The visual color effect of offset printing products is obtained in a known manner by an interaction of subtractive and additive color mixing. Individual half-tone dots of the various printing inks are printed in differing size both next to one another, and one above the other so as to overlap to a greater or lesser extent. At the same time, the printing inks used have a glazing effect, that is to say the effect corresponds to a filter resting on the white printing material. The coloring direction of the composite printing of the half-tone dots is determined both by the layer thickness of the applied printing ink and by the size of the half-tone dots (geometrical surface covering). Thus, by adjusting the ink-guide members in the individual printing units, the color location of a printing-image point can be varied. Typically, in color printing, three colored inks, cyan, magenta, and yellow, are utilized along with the standard black printing ink for printing color images.
It is well known in the printing art to record the ink application on a printing product photoelectrically by means of extra co-printed measuring elements, and to derive from this a measure of the applied ink quantity. This is usually carried out by means of densitometers, since there is a relatively simple relation between the ink-density value and the layer thickness of the ink, and therefore also the position of the ink-guide members designed, for example, as ink slides. However, a densitometric recording of the ink guidance does not provide for any numerical judgement with respect to visual color perception. Furthermore, the disadvantage of the extra co-printed measuring elements is that only the desired ink application for these measuring fields is controlled. The color effect in the printed image itself is completely ignored and is accordingly also varied only indirectly.
A method for assessing the printing quality and for regulating the ink guidance is known, for example, from EP 0,143,744 A1. The reflectances in four spectral regions are measured with one or more measuring heads by means of image elements in the subject. For the three colored inks, the spectral regions are selected in such a way that the conventional ink-density values are obtained. For the black printing ink, the spectral reflectance in the infrared region is determined. The corresponding surface coverings are determined (unmasked) from these reflectances or ink-density values by using the Neugebauer equations. This is carried out at the same image points of copies printed on the machine and also on a desired original. Regulating commands for the ink guidance of the printing machine are then derived from a desired/actual comparison of the surface coverings.
DE 4,311,132 A1 discloses a method for ink regulation/control in a printing machine. The method is characterized in that the density spectra of specimen prints of the individual printing inks participating in the composite printing and having a predetermined surface covering, and of the white paper are recorded and stored, the density spectra of a measuring point of the original and of a measuring point of the printed copy are recorded, and in that these measured density spectra of the original and of the printed copy are each represented as a linear combination of the factor-weighted density spectra of the individual printing inks and of the white paper with the aim of representing the density spectra of the composite printing of the measuring point of the original and printed copy in the best possible way in each case by means of this linear combination. The ink feed is changed as a function of the difference between the individual factors of the respective linear combination (original/printed copy) which are interpreted as degrees of surface covering.
A disadvantage of this known method is that the factors in the formulation of the linear combination for representing the entire density spectrum (composite printing) are formulated from the density spectra of the individual inks as degrees of surface covering. Where the factors are concerned, a formulation of this kind presents problems in the case of high degrees of surface covering of a particular ink at the image point and fails completely when one or more inks are printed in full tone. The reason lies, in a way which is simple to see, in the fact that, for example, if an ink of the original is printed as a full-tone surface, the corresponding ink in the printed copy can likewise be printed only as a full tone and not to a greater extent than this. It is therefore impossible to regulate inks in the full-tone range.